1. Field of the Invention
The present invention relates to a marine propulsion device.
2. Description of the Related Art
A type of marine propulsion device including a catalyst, a cooling pathway, a pilot pathway, a restriction valve, and a thermostat is known in the art (see Japan Laid-open Patent Application Publication No. 2014-163288). The catalyst is disposed in an exhaust pathway connected to an engine. The cooling pathway is disposed in the surroundings of the engine and the exhaust pathway. The pilot pathway is disposed above the cooling pathway and is connected to the cooling pathway. The restriction valve is disposed in the pilot pathway. The thermostat is disposed downstream of the cooling pathway. In this type of marine propulsion device, when the amount of cooling water supplied to the cooling pathway is reduced, the restriction valve prevents the fluid from reversely flowing from the pilot pathway to the cooling pathway. With this configuration, the speed of discharging the cooling water in the surroundings of the catalyst is decelerated. Hence, degradation in catalyst cooling performance is prevented.
However, when the amount of cooling water supplied to the cooling pathway is reduced during driving of the engine in the above-described type of marine propulsion devices, the cooling water in the surroundings of the cylinders increases in temperature and therefore the thermostat is normally kept opened. With this mechanism, the speed of discharging the cooling water in the surroundings of the catalyst is decelerated as described above, but the speed of discharging the cooling water in the surroundings of the cylinders of the engine is not effectively decelerated.
Additionally, it is required to control cylinder cooling performance not only when the amount of the cooling water supplied to the cooling pathway is reduced but also during normal operation of the marine propulsion devices.